1. Field of the Invention
The present invention relates to heat exchangers, and more particularly to closed circuit evaporative heat exchangers and combined direct and indirect closed circuit evaporative heat exchangers.
2. Description of the Prior Art
Waste heat may be rejected to the atmosphere by dry or sensible heat exchangers. In a dry or sensible heat exchanger, there are two fluids: an air stream and a process fluid stream. In a closed system, the process fluid stream is enclosed so that there is no direct contact between the air stream and the process fluid stream; the process fluid stream is not open to the atmosphere. The enclosing structure may be a coil of tubes. Sensible heat is exchanged as the air stream is passed over the structure enclosing the process fluid stream. In the art these structures are known as "compact heat exchangers."
In most climates, evaporative heat exchangers offer significant process efficiency improvements over dry heat exchangers. One type of evaporative heat exchanger is a direct evaporative heat exchanger. In a direct heat exchanger, only an air stream and an evaporative liquid stream are involved; the evaporative liquid stream is usually water, and the two streams come into direct contact with each other.
Another type of evaporative heat exchanger is an indirect closed circuit evaporative heat exchanger, where three fluid streams are involved: an air stream, an evaporative liquid stream, and an enclosed process fluid stream. The enclosed fluid stream first exchanges sensible heat with the evaporative liquid through indirect heat transfer, since it does not directly contact the evaporative liquid and then the air stream and the evaporative liquid exchange heat and mass when they contact each other.
Another type of evaporative heat exchanger is a combined direct and indirect closed circuit evaporative heat exchanger. Examples of combined systems are disclosed in U.S. Pat. Nos. 5,435,382 (1995) and 5,816,318 (1998) to Carter.
Both dry and evaporative heat exchangers are commonly used to reject heat as coolers or condensers. Evaporative coolers reject heat at temperatures approaching the lower ambient wet bulb temperatures, while dry coolers are limited to approaching the higher ambient dry bulb temperatures. In many climates the ambient wet bulb temperature is often 20 to 30.degree. F. below the ambient design dry bulb temperature. Thus, in an evaporative cooler, the evaporative liquid stream may reach a temperature significantly lower than the ambient dry bulb temperature, offering the opportunity to increase the efficiency of the cooling process and to lower the overall process energy requirements. Evaporative condensers offer similar possibilities for increased efficiency and lower energy requirements. In spite of these opportunities to increase process efficiencies and lower overall process energy requirements, evaporative cooling and evaporative condensing are often not used due to concern about water consumption from evaporation of the evaporative liquid and freezing potentials during cold weather operation.
In addition, both sensible and evaporative heat exchangers are typically sized to perform their required heat rejection duty at times of greatest thermal difficulty. This design condition is typically expressed as the summer design wet bulb or dry bulb temperature. While it is often critical that the heat rejection equipment be able to reject the required amount of heat at these design conditions, the duration of these elevated atmospheric temperatures may account for as little as 1% of the hours of operation of the equipment. The remainder of the time, the equipment may have more capacity than required, resulting in the waste of energy and evaporative liquid.